Pseudostratified Columnar Epithelium:
Pseudostratified columnar epithelium is confusing at first, because under a microscope it looks
as if it’s layered. That’s because adjacent cells often have their nuclei at different levels, and this creates the illusion that they are layered. If you look carefully though, you can see that each cell touches the basal lamina, and so the cells are not layered.
Pseudostratified Epithelium in the Trachea (Windpipe)
Note the presence of cilia and of mucus-secreting goblet cells.
Pseudostratified columnar epithelium, like simple columnar epithelium, is typically found lining body tubes. It lines the respiratory passages, for instance, and much of the reproductive tract.
Pseudostratified columnar epithelium often has hairlike projections called cilia
that extend into the lumen of the tubes they line. These cilia beat back-and-forth and can move substances through the tubes. For instance, the ciliated cells lining the female reproductive tract help create currents in the fluid filling the tract; these currents move an ovum from the ovary through the oviduct and into the uterus. Ciliated cells lining the respiratory passages help to prevent mucus from clogging the tubes and also help to sweep dust particles, microbes, and other potentially harmful substances out of the air as it passes through the tubules.
Pseudostratified columnar epithelium that is specialized for mucus production typically contains specialized cells known as goblet cells
. Goblet cells are specialized for secreting mucus, and are named for their very distinctive shape.
Stratified Squamous Epithelium:
Stratified squamous epithelium, as you have probably guessed, consists of several layers of flattened cells. It forms a thick, protective layer, and makes up the outer portion of the skin. The tongue and the esophagus, like the skin, are subject to a great deal of abrasion, and so the outer surface of the tongue and the inner surface of the esophagus consist of stratified squamous epithelium. The lining of the vagina also consists largely of stratified squamous epithelium. The cornea of the eye is another place where you can find stratified squamous epithelium.
Stratified Squamous Epithelium in the Tongue
It’s an imperfect world, and just because a tissue is classified as “stratified squamous epithelium” doesn’t mean that every cell in it is flat. Typically in stratified squamous epithelium, the uppermost cells are squamous in shape, while those closer to the basal lamina are more cuboidal.
Stratified Cuboidal Epithelium:
Stratified cuboidal epithelium lines some of the larger ducts in the body, where it’s important to have a thick and (more or less) waterproof layer of cells. Stratified cuboidal epithelium lines the larger ducts of the mammary glands, some of the sweat glands, and the salivary ducts.
Stratified Cuboidal Epithelium Lining the Lumen of a Dog’s Mammary Gland
Stratified Columnar Epithelium:
Stratified columnar epithelium is quite rare, but it does form a thick layer of cells lining the largest tubules in the mammary glands and the salivary glands.
Stratified Columnar Epithelium Lining a Body Tube
Transitional epithelium is found lining the urinary bladder, and the cells of this tissue are specialized to change shape in response to pressure. When the bladder is empty, these cells are more or less cuboidal in shape, but as the bladder fills the cells become compressed and flattened.
Transitional Epithelium in the Urinary Bladder
Glands and Glandular Epithelium:
Glandular epithelium is named not for the cells’ shapes or arrangements, but for what the cells do. Glands
are organs that secrete substances into the blood, or into body cavities or ducts – and glandular epithelium lines the glands. More precisely, glandular cells are those which produce chemical substances (proteins, lipids, etc.) that are not used by the cells themselves, but are instead stored and eventually secreted to be used by other parts of the organism.
Broadly speaking, there are two types of glands, endocrine glands
and exocrine glands
. Endocrine glands have no ducts, and secrete the substances they produce (called hormones
) directly into the blood, which transports the hormones to where they’re needed. We’ll consider endocrine glands in some detail when we discuss the endocrine system.
Exocrine glands, by contrast, secrete their products into ducts
that transport them onto body surfaces or into body cavities. The exocrine glands are classified according to how they produce their products.
The cells of merocrine glands
secrete watery, protein-rich fluids through exocytosis
. Most glandular cells are merocrine in nature, and merocrine glands include some (but not all) of the sweat glands. The salivary glands are also merocrine glands.
If the substance produced by merocrine cells is thin and watery, it is known as serous
fluid. For example, serous fluid is secreted by the serous membranes
) lining the body cavities; the serous fluid produced by these membranes helps to lubricate the internal organs so that they don’t damage each other as they rub together.
If the substance produced by merocrine cells is thicker and contains relatively large amounts of the protein mucin
, it is known as mucus
. Mucus in the respiratory passages helps to trap foreign particles and pathogens in the air that would otherwise damage or infect lung tissues, and mucus in the alimentary canal helps protect the linings of the stomach and intestines against digestive enzymes and acids.
The cells of apocrine glands
lose small portions of their cytoplasm during secretion. Basically, the portion of a cell that is closest to the lumen of the duct ruptures, releasing its contents into the duct – but the remainder of the cell survives. (In apo
crine glands, only the apex
of the cell ruptures.) Some of the sweat glands are apocrine glands, as are the mammary glands. (So milk is made of ruptured mammary gland cells.)
The cells of holocrine glands
rupture completely during secretion, and so the substances produced by holocrine glands consist of the remains of ruptured cells. (In holo
crine glands, the whole
cell ruptures.) The sebaceous glands of the skin that secrete an oily, waterproofing substance are holocrine glands.
Epithelial Tissues and Cancer:
Epithelial tissues form the boundary between your body and the external environment – whether it’s the skin that covers your body or the epithelial tissues that line your respiratory, reproductive, and digestive passages. As such, epithelial tissues are your first line of defense against pathogenic organisms, noxious chemicals, ultraviolet radiation, and any other substances that might cause damage to the body.
Perhaps it’s not too surprising, therefore, that something like 90% of the cancers that afflict us occur in epithelial tissues – skin cancers, cancers of the tissues lining the lungs, cancers of the stomach and intestines, and so forth. (Cancer of epithelial tissues is known as carcinoma
.) Not only is epithelium directly exposed to noxious agents to a far greater extent than are other body tissues, but the cells of epithelial tissues are fast-growing and rapidly-dividing, which may make them more prone to become cancerous than are most body cells.